I chose the tru-cool stacked plate for a couple reasons
1. stacked plate design results in lower flow restriction as compared to tube & fin designs.
2. the 4590 simply because of suggestions from others indicating that it would provide more than adequate cooling for thier tranny even when towing heavy in conditions that will normall shoot the temperature through the roof.
I'll echo what BJS says, the low pressure drop design of the stacked plate is what drew me. The 4590 is rated for 28,000 lbs which should be adequate for most anything you put behind or in the truck. The 4739 is an even larger unit and some folks are running them rather than the 4590. I picked my 4590 up off e-bay from some tranny shop out in Oregon.